highway engineering-ii · dr. muhammad ashraf javid assistant professor department of civil and...

HIGHWAY ENGINEERING-II

Dr. Muhammad Ashraf JavidAssistant Professor

Department of Civil and Environmental EngineeringUniversity of Nizwa, Sultanate of Oman

Email: [email protected]

Asphalt-Aggregate Mixes

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HMA

� Mix Design• Philosophy• Weight-Volume Relationships• Methods

• Marshall• Superpave (not covered)

� Evaluation

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HMA

Aggregate

Asphalt

Air Voids

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HMA

� The two key components of pavement design are • Mix design• Structural design (i.e. pavement design part of thi s course)

� The goal of mix design is to determine the optimum mixture of component materials for a given application.

� This includes detailed evaluations of � Aggregate� Asphalt � Determination of their optimum blending ratios.

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HMA Design Objectives� Deformation Resistance (stability). HMA should not distort (rut) or

deform (shove) under traffic loading. HMA deformation is re lated to one or more of the following:

� Aggregate surface and abrasion characteristics. Rounded particles tend to slip by one anothercausing HMA distortion under load while angular particles i nterlock with one another providing agood deformation resistant structure. Brittle particles c ause mix distortion because they tend tobreak apart under agitation or load.

� Aggregate gradation. Gradations with excessive fines (either naturally occurr ing or caused byexcessive abrasion) cause distortion because the large amo unt of fine particles tend to push thelarger particles apart and act as lubricating ball-bearing s between these larger particles. Agradation resulting in low VMA or excessive asphalt binder c ontent can have the sameeffect. Gradation specifications are used to ensure accept able aggregate gradation.

� Asphalt binder content. Excess asphalt binder content tends to lubricate and push a ggregateparticles apart making their rearrangement under load easi er. The optimum asphalt bindercontent as determined by mix design should prevent this.

� Asphalt binder viscosity at high temperatures. In the hot summer months, asphalt binderviscosity is at its lowest and the pavement will deform more e asily under load. Specifying anasphalt binder with a minimum high temperature viscosity (a s can be done in the Superpaveasphalt binder selection process) ensures adequate high te mperature viscosity.

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HMA Design Objectives� Fatigue resistance.� HMA should not crack when subjected to repeated loads over ti me. HMA fatigue

cracking is related to asphalt binder content and stiffness . Higher asphalt bindercontents will result in a mix that has a greater tendency to de form elastically (or at leastdeform) rather than fracture under repeated load. The optim um asphalt binder contentas determined by mix design should be high enough to prevent e xcessive fatiguecracking. The use of an asphalt binder with a lower stiffness will increase a mixture'sfatigue life by providing greater flexibility. However, th e potential for rutting must alsobe considered in the selection of an asphalt binder.

� Note that fatigue resistance is also highly dependent upon t he relationship betweenstructural layer thickness and loading. However, this sect ion only addresses mix designissues.

� Low temperature cracking resistance

� HMA should not crack when subjected to low ambient temperatu res. Low temperaturecracking is primarily a function of the asphalt binder low te mperaturestiffness. Specifying asphalt binder with adequate low tem perature properties (as canbe done in the Superpave asphalt binder selection process) s hould prevent, or at leastlimit, low temperature cracking.

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HMA Design Objectives� Durability� HMA should not suffer excessive aging during production and service

life. HMA durability is related to one or more of the followin g:

� The asphalt binder film thickness around each aggregate particle. If the filmthickness surrounding the aggregate particles is insuffic ient, it is possible thatthe aggregate may become accessible to water through holes i n the film. If theaggregate is hydrophilic, water will displace the asphalt f ilm and asphalt-aggregate cohesion will be lost. This process is typically r eferred to asstripping. The optimum asphalt binder content as determine d by mix designshould provide adequate film thickness.

� Air voids. Excessive air voids (on the order of 8 percent or more) incre aseHMA permeability and allow oxygen easier access to more asph alt binder thusaccelerating oxidation and volatilization. To address this , HMA mix designseeks to adjust items such as asphalt content and aggregate g radation toproduce design air voids of about 4 percent. Excessive air vo ids can be eithera mix design or a construction problem and this section only a ddresses themix design problem.

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HMA Design Objectives� Moisture Damage Resistance

� HMA should not degrade substantially from moisture penetra tion into themix. Moisture damage resistance is related to one or more of t he following:

� Aggregate Mineral and Chemical Properties� Some aggregates attract moisture to their surfaces, which c an cause

stripping. To address this, either stripping-susceptible aggregates can beavoided or an anti-stripping asphalt binder modifier can be used.

� Air Voids (same as for durability of mix)� When HMA air voids exceed about 8 percent by volume, they may b ecome

interconnected and allow water to easily penetrate the HMA a nd causemoisture damage through pore pressure or ice expansion. To a ddress this,HMA mix design adjusts asphalt binder content and aggregate gradation toproduce design air voids of about 4 percent. Excessive air vo ids can be eithera mix design or a construction problem and this section only a ddresses themix design problem.

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HMA Design Objectives

� Skid Resistance

� HMA placed as a surface course should provide sufficient fri ction when incontact with a vehicle's tire. Low skid resistance is genera lly related to one ormore of the following:

• Aggregate characteristics such as texture, shape, size and resistance topolish. Smooth, rounded or polish-susceptible aggregates are les s skidresistant. Tests for particle shape and texture can identif y problemaggregate sources. These sources can be avoided, or at a mini mum,aggregate with good surface and abrasion characteristics c an be blendedin to provide better overall characteristics.

• Asphalt binder content. Excessive asphalt binder can cause HMAbleeding. Using the optimum asphalt binder content as deter mined by mixdesign should prevent bleeding.

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HMA Design Objectives� Workability� HMA must be capable of being placed and compacted with reason able effort. Workability is

generally related to one or both of the following:� Aggregate texture, shape and size. Flat, elongated or angular particles tend to interlock

rather than slip by one another making placement and compact ion more difficult (noticethat this is almost in direct contrast with the desirable agg regate properties for deformationresistance). Although no specific mix design tests are avai lable to quantify workability,tests for particle shape and texture can identify possible w orkability problems.

� Aggregate gradation. Gradations with excess fines (especially in the 0.60 to 0.3 0 mm (No.30 to 50) size range when using natural, rounded sand) can cau se a tender mix. Agradation resulting in low VMA or excess asphalt binder cont ent can have the sameeffect. Gradation specifications are used to ensure accept able aggregate gradation.

� Asphalt binder content. At laydown temperatures (above about 120 °C (250 °F)) asphaltbinder works as a lubricant between aggregate particles as t hey are compacted. Therefore,low asphalt binder content reduces this lubrication result ing in a less workable mix. Notethat a higher asphalt binder content is generally good for wo rkability but generally bad fordeformation resistance.

� Asphalt binder viscosity at mixing/laydown temperatures. If the asphalt binder viscosity istoo high at mixing and laydown temperatures, the HMA becomes difficult to dump, spreadand compact. The Superpave rotational viscometer specific ally tests for mixing/laydowntemperature asphalt binder viscosity.

� Knowing these objectives, the challenge in mix design is the n to develop a relatively simpleprocedure with a minimal amount of tests and samples that wil l produce a mix with all theabove HMA qualities.

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HMA Mix Design� Mix design is a laboratory simulation

� Mix design is meant to simulate actual HMA• Manufacturing• Construction and • Performance• to the extent possible

� From this simulation we can predict (with reasonable certainty) what type of mix design is best for the particular application in question and how it will perform.

� Being a simulation, mix design has its limitations. Specifically, there are substantial differences between laboratory and field conditions.

� Sample Size� Compaction Method� ………………..

� However, despite limitations such as the preceding, mix design procedures can provide a cost effective and reasona bly accurate simulation that is useful in making mix design deci sions. 12

Weight-Volume Relationships� Calculation of both weight and volume relationships for

asphalt concrete mixtures are required.

� A basic understanding of weight-volume relationships ofcompacted asphalt concrete mixtures is important fromboth a mixture design standpoint and from a fieldconstruction standpoint.

� If asphalt cement is not absorbed, the calculations arerelatively straightforward that the bulk specific gravity ofthe aggregate can be used to estimate the volume ofaggregate.

� If asphalt cement absorption is identical to waterabsorption, the calculations are again relativelystraightforward in that the apparent specific gravity of th eaggregate can be used to calculate the volume ofaggregate.

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Weight-Volume Relationships

� AGGREGATES• Specific Gravities ?

� AGGREGATE• Solid Portion• Voids• Impermeable• Water Permeable Only• Water and Asphalt Permeable

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Weight -Volume Relationships

� Apparent Specific Gravity

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Weight-Volume Relationships

� Bulk (Dry) Specific Gravity

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Weight-Volume Relationships� Bulk (SSD) Specific Gravity

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Weight-Volume Relationships� Effective Specific Gravity

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Weight-Volume Relationships� Maximum Theoretical Specific Gravity

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Weight-Volume Relationships� Voids in Mineral Aggregate

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Volumetric Properties of Asphalt Mixtures

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� Air Voids, V a - the total volume of the small pockets of air between the coated aggregate particles throughout a compacted paving mixture, expressed as percent of the total volume of the compacted paving mixture.

� Voids in the Mineral Aggregate, VMA - the volume of inter-granular void space between the aggregate particles of a compacted paving mixture that includes the air voids and the effective asphalt content, expressed as a percent of the total volume of the compacted paving mixture.

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• Voids Filled with Asphalt, VFA - the percentage portion of the volume of inter-granular void space between the aggregate particles that is occupied by the effective asphalt. It is expressed as the ratio of (VMA - V a) to VMA.

• Asphalt Content, P b - the total asphalt content of a paving mixture

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• Effective Asphalt Content, P be -the total asphalt content of a paving mixture minus the portion of asphalt absorbed into the aggregate particles.

• Absorbed Asphalt Content, P ba -the portion of asphalt absorbed into the aggregate particles.

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Mixtures Specific Gravities

� Two measurements of the specific gravity of the HMA mixture are important in determining the volumetric propert ies of the HMA: the maximum theoretical specific gravity, G mm, and bulk specific gravity, G mb.

� Maximum Theoretical Specific Gravity, G mm - the ratio of the mass in air of a unit volume of the asphalt and agg regate in the mixture at a stated temperature to the mass in air of equal density of an equal volume of gas-free distilled wa ter at a stated temperature. In other words, the maximum th eoretical specific gravity, G mm, is the mass of the asphalt and aggregate mixture divided by the volume, not including the ai r voids.

� Bulk Specific Gravity, G mb - the ratio of the mass in air of a unit volume of the compacted asphalt and aggregate mixture at a stated temperature to the mass in air of equal density of an equal volume of gas-free distilled water at a st ated temperature. In other words, the bulk specific gra vity, G mb, is the mass of the asphalt and aggregate mixture divid ed by the volume, including the air voids.

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HMA Design Procedure (Marshall Method)

HMA Mix Design Procedure

� General Steps

� Selection of Aggregate� Selection of Asphalt Binder� Determination of Optimum Asphalt Content

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HMA Mix Design Procedure

� Types

� Old• Marshall• Hveem

� New• Superpave

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HMA Mix Design - Marshall

� The basic concepts of the Marshall mix design method were originally developed by Bruce Marshall of the Mississippi Highway Department around 1939 and then refined by the U.S. Army.

� Currently, the Marshall method is used in some capacity by about 38 states of USA and most parts of the world.

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� Simulation of

• Mixing and Laying• Compaction• In-service Behavior

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HMA Mix Design - Marshall� The Marshall mix design method consists of

6 basic steps• Aggregate selection

• Asphalt binder selection

• Sample preparation (including compaction)

• Stability determination using the Marshall stability and flow test

• Density and voids calculations

• Optimum asphalt binder content selection

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HMA Mix Design - Marshall� Aggregate Selection

• Physical

• Chemical

• Mineralogical

• Gradation ?

32http://www.pavementinteractive.org/article/gradation-and-size/


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� Maximum Density Curve• In 1962 FHWA published a modified version of

Fuller’s equation with a different exponent

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HMA Mix Design - Marshall� Maximum Density Curve

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� Gradation ?

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Maximum size. The smallest sieve through which 100 percent of the aggregate sample particles pass. Superpave defines the maximum aggregate size as “one sieve larger than the nominal maximum size” .Nominal maximum size. The largest sieve that retains some of the aggregate particles but generally not more than 10 percent by weight. Superpave defines nominal maximum aggregate size as “one sieve size larger than the first sieve to retain more than 10 percent of the material”


� Gradation ?

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Passing Sieve Designation (Percent

by weight)

Retained on Sieve Designation (Percent

by weight)

AGGREGATE TYPE

COARSE AGGREGATES

FINE AGGREGATE

S

MINERAL FILLER

¾in.(19.0 mm) ½in.(12.5 mm) 5 —- —-

½in.(12.5 mm) 3/8in.(9.5mm) 32 —- —-

3/8in.(9.5mm) No.4(4.75mm) 37 —- —-

No.4(4.75mm) No.10(2.00mm) 22 7 —-

No.10(2.00mm) No.40(0.475mm) 4 28 —-

No.40(0.475mm) No.80(0.177mm) —- 39 5

No.80(0.177mm) No.200(0.75mm) —- 24 30

No.200(0.75mm) —– —- 2 65

Total 100 100 100

SIEVE ANALYSIS OF AGGREGATES (PERCENTAGE USED FOR E XPERIMENT)


� Binder Selection

• Penetration Grade !!

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HMA Mix Design - Marshall� Mix and Compaction

� Impact

� Hammer Blows

� 35, 50, 75

� on both sides of sample

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HMA Mix Design - Marshall� Stability and Flow

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HMA Mix Design - Marshall� Calculations

• Air Voids (%)

• Density of Specimens

• Voids in Mineral Aggregates (VMA)

• Voids Filled with Asphalt (VFA)

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HMA Mix Design - Marshall� Plots

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Typical Marshall Design Criteria (from Asphalt Institute, 1979

Mix Criteria

Light Traffic(< 104 ESALs)

Medium Traffic(104 – 106 ESALs)

Heavy Traffic(> 106 ESALs)

Min. Max. Min. Max. Min. Max.

Compaction

(number of blows on each end

of the sample)

35 50 75

Stability (minimum)

2224 N(500 lbs.)

3336 N(750 lbs.)

6672 N(1500 lbs.)

Flow (0.25 mm (0.01

inch))8 20 8 18 8 16

Percent Air Voids

3 5 3 5 3 5

Typical Marshall Minimum VMA(from Asphalt Institute, 1979

Nominal MaximumParticle Size Minimum VMA (percent)

(mm) (U.S.)

63 2.5 inch 11

50 2.0 inch 11.5

37.5 1.5 inch 12

25.0 1.0 inch 13

19.0 0.75 inch 14

12.5 0.5 inch 15

9.5 0.375 inch 16

4.75 No. 4 sieve 18

2.36 No. 8 sieve 21

1.18 No. 16 sieve 23.5

Summary of Weight -Volume Relationships

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Example� An asphalt mixture has a bulk-specific gravity ( Gmb) of

2.329. The phase diagram in Figure shows five properties(four specific gravities and the asphalt content) of acompacted specimen of HMA that have been measured at25◦C. Using only these values, find all the volumetricproperties and mass quantities as indicated on thecomponent diagram .

Example

Solution to Example

highway engineering-ii · dr. muhammad ashraf javid assistant professor department of civil and...

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